If $\cos^{-1}\left({2\over3x}\right)+\cos^{-1}\left({3\over4x}\right)={\pi\over3}\quad\left(x>{3\over4}\right)$ then x is equal to :
(1) $\sqrt{145}\over12$
(2) $\sqrt{145}\over10$
(3) $\sqrt{146}\over12$
(4) $\sqrt{145}\over11$
(1) $\sqrt{145}\over12$
(2) $\sqrt{145}\over10$
(3) $\sqrt{146}\over12$
(4) $\sqrt{145}\over11$